This invention relates generally to oil well monitor systems, and more particularly to remotely controlled downhole switching apparatus for use in such systems.
There are many industrial installations in which it is desirable to selectively connect remotely disposed, and often inaccessible, devices to a cable by means of control signals impressed upon the cable. In oil well monitor systems, for example, in which a downhole instrumentation package is used to monitor various physical parameters in the downhole environment, such as temperature and pressure, it is common practice to employ the cable which carries power to a downhole AC motor driving a submersible pump to carry DC signals between the downhole instrumentation package and a control and readout unit on the surface. Normally, a three-phase, Y-connected motor is employed in such installations, and the DC signals are coupled to the neutral of a three-phase power source driving the cable and from the neutral of the motor to the instrumentation package. In order to monitor the condition of the downhole motor, it is desirable to measure periodically the motor/cable system insulation to ground resistance, since this gives an early indication of impending failure. Such measurements require the use of relatively high voltages, e.g., 500 volts or more. However, since the downhole instrumentation package transducers and their associated circuits generally have a much lower impedance to ground than the motor/cable insulation system, it is necessary to disconnect the instrumentation package from the cable prior to testing, and, subsequently, to reconnect the instrumentation package when testing has been completed. Accordingly, various downhole latch/delatch systems have been proposed.
One such system is disclosed in U.S. Pat. No. 4,178,579 to McGibbeny, et al., issued Dec. 11, 1979, and assigned to the same assignee as the present invention. In the McGibbeny, et al. system, the downhole instrumentation package is selectively connected to the neutral of the three-phase motor by a reed switch, which may be opened by the application of a negative voltage of a predetermined magnitude to the cable. The negative voltage causes a Zener diode to break down and allows current to flow in a delatch coil, opening the reed switch. The magnetic flux produced by the delatch coil is sufficient to overcome the flux of a permanent magnet which, while of insufficient magnitude to close the reed switch, is of sufficient magnitude to hold the reed switch in closed position once it is closed. To relatch the reed switch after it has been opened, a latch coil, whose magnetic flux adds to that of the permanent magnet, is energized by the rectified AC current flowing in the secondary of a current transformer whose primary is one of the three-phase lines to the motor.
Although the McGibbeny, et al. latch/delatch system performs satisfactorily, it employs a relatively large number of components and is relatively complex, expensive and bulky. Moreover, because of the high temperature/high pressure downhole environment, the components, particularly the semiconductor components, of such systems are more subject to failure. Therefore, in systems such as the McGibbeny, et al. system, it is difficult to achieve the desired degree of reliability. Many prior downhole switching systems suffer from these disadvantages.